Normal human mouth contains lots of bacteria, fungi, and even viruses, where bacteria are the major population. There are 100 million bacteria in one milliliter of saliva, and the whole mouth contains more than 600 kinds of bacteria. Not all of these bacteria are pathogens. Certain probiotics are also present. These bacteria maintain a relative balance and are non-pathogenic under normal condition. Under the situation of low immunity or resistance, oral environmental changes, medication, or systematic disease, over growth of pathogens result in oral diseases, as minor as bad breath, dental plaque, gingivitis or as severe as cavity, periodontal disease and even bacteremia caused by massive bacteria reproduction after blood vessels infection.
According to the yearly National Health Insurance statistic analysis by Taiwan Dental Association, 90% adults in Taiwan area suffering periodontal diseases demonstrated the importance of oral hygiene.
There are two kinds of periodontal diseases, i.e., gingivitis and periodontitis. The major symptoms of gingivitis are gum bleeding, swelling, redness and etc. Periodontitis indicates the status of destroyed alveolar bone which supports gum tissue and teeth.
One of the periodontal disease symptoms is deepen grove between teeth and gum resulting in periodontal pocket. The periodontal pocket of people with healthy teeth is around 1 to 2 mm, which is 3 to 4 mm of mild periodontal disease patients, 4 to 6 mm of moderate patients, and over 6 mm of severe patients. Along with deeper periodontal pocket, the gum is getting shorter. The appearance of longer or mobile teeth could be the warning sign of severe periodontal diseases. Gingivitis and periodontitis are caused by periodontal bacteria infection. The representative bacterium is Porphyromonas gingivalis. 
Glycoprotein of saliva forms a film on the teeth surface which enables bacteria attachment. After bacteria utilizing the sugar of food, the film is getting bigger and thicker resulting in dental plaque. Moreover, if the dental plaque is formed in periodontal pocket, periodontal bacteria will reproduce massively in the dental plaque. Due to anaerobic characteristics of periodontal bacteria, the oxygen unreachable periodontal pocket is their most optimal growth condition. Dental plaque is also the incubator of Streptococcus mutans. More than 90% adults carry this type of bacteria in their mouth, which is also the major bacteria strain causing cavity. When acidic substances generated from sugar degradation by Streptococcus mutans erode enamel and dentin, it will cause cavity.
Periodontal bacteria invading gum will cause immunological reactions. Periodontal bacteria secret enzymes dissolving gum cell and invading the internal side of the gum. With minimal level of periodontal bacteria, the invasion can be stopped. As long as massive reproduction is established, the situation can not be controlled.
There are some present methods to prevent the formation of cavity or periodontal diseases, such as applying anti-adhesives to avoid bacteria attachment on teeth surface, and to minimize dental plaque formation, and bacteria erosion of dentin (R.O.C. patent application number 094144377); applying anti-bacterial reagent to inhibit bacteria growth (U.S. Pat. No. 5,368,845; WO 92/14475); or widely used fluoride to reduce the solubility of enamel against acidic substances for cavity prevention. However, studies of Lactobacillus fermentum and its oral disease curative effects are rarely seen.